The present invention relates to multipole ion guides for use in mass spectrometry. The apparatus and methods for ionization described herein are enhancements of the techniques that are referred to in the literature relating to mass spectrometry—an important tool in the analysis of a wide range of chemical compounds. Specifically, mass spectrometers can be used to determine the molecular weight of sample compounds. The analysis of samples by mass spectrometry consists of three main steps—formation of gas phase ions from sample material, mass analysis of the ions to separate the ions from one another according to ion mass, and detection of the ions. A variety of means and methods exist in the field of mass spectrometry to perform each of these three functions. The particular combination of the means and methods used in a given mass spectrometer determine the characteristics of that instrument.
To mass analyze ions, for example, one might use magnetic (B) or electrostatic (E) analysis. Ions passing through a magnetic or electrostatic field will follow a curved path. In a magnetic field the curvature of the path will be indicative of the momentum-to-charge ratio of the ion. In an electrostatic field, the curvature of the path will be indicative of the energy-to-charge ratio of the ion. If magnetic and electrostatic analyzers are used consecutively, then both the momentum-to-charge and energy-to-charge ratios of the ions will be known and the mass of the ion will thereby be determined. Other mass analyzers are the quadrupole (Q), the ion cyclotron resonance (ICR), the time-of-flight (TOF), and the quadrupole ion trap analyzers. The analyzer which accepts ions from the ion guide described here may be any of a variety of these.
Before mass analysis can begin, however, gas phase ions must be formed from sample material. If the sample material is sufficiently volatile, ions may be formed by electron ionization (EI) or chemical ionization (CI) of the gas phase sample molecules. For solid samples (e.g., semiconductors, or crystallized materials), ions can be formed by desorption and ionization of sample molecules by bombardment with high energy particles. Secondary ion mass spectrometry (SIMS) , for example, uses keV ions to desorb and ionize sample material. In the SIMS process a large amount of energy is deposited in the analyte molecules. As a result, fragile molecules will be fragmented. This fragmentation is undesirable in that information regarding the original composition of the sample (e.g., the molecular weight of sample molecules) will be lost.
For more labile, fragile molecules, other ionization methods now exist. The plasma desorption (PD) technique was introduced by Macfarlane et al. in 1974 (Macfarlane, R. D.; Skowronski, R. P.; Torgerson, D. F., Biochem. Biophys. Res Commoun. 60 (1974) 616). Macfarlane et al. discovered that the impact of high energy (MeV) ions on a surface, like SIMS would cause desorption and ionization of small analyte molecules. However, unlike SIMS, the PD process results also in the desorption of larger, more labile species (e.g., insulin and other protein molecules).
Lasers have been used in a similar manner to induce desorption of biological or other labile molecules. See, for example, VanBreeman, R. B.: Snow, M.: Cotter, R. J., Int. J. Mass Spectrom. Ion Phys. 49 (1983) 35; Tabet, J. C.; Cotter, R. J., Anal. Chem. 56 (1984) 1662; or Olthoff, J. K.; Lys, I.: Demirev, P.: Cotter, R. J., Anal. Instrument. 16 (1987) 93. Cotter et al. modified a CVC 2000 time-of-flight mass spectrometer for infrared laser desorption of involatile biomolecules, using a Tachisto (Needham, Mass.) model 215G pulsed carbon dioxide laser. The plasma or laser desorption and ionization of labile molecules relies on the deposition of little or no energy in the analyte molecules of interest. The use of lasers to desorb and ionize labile molecules intact was enhanced by the introduction of matrix assisted laser desorption ionization (MALDI) (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshica, T., Rapid Commun. Mass Spectrom. 2 (1988) 151 and Karas, M.; Hillenkamp, F., Anal. Chem. 60 (1988) 2299). In the MALDI process, an analyte is dissolved in a solid, organic matrix. Laser light of a wavelength that is absorbed by the solid matrix but not by the analyte is used to excite the sample. Thus, the matrix is excited directly by the laser, and the excited matrix sublimes into the gas phase carrying with it the analyte molecules. The analyte molecules are then ionized by proton, electron, or cation transfer from the matrix molecules to the analyte molecules. This process, MALDI, is typically used in conjunction with time-of-flight mass spectrometry (TOFMS) and can be used to measure the molecular weights of proteins in excess of 100,000 daltons.
Atmospheric pressure ionization (API) includes a number of methods. Typically, analyte ions are produced from liquid solution at atmospheric pressure. One of the more widely used methods, known as electrospray ionization (ESI), was first suggested by Dole et al. (M. Dole, L. L. Mack, R. L. Hines, R. C. Mobley, L. D. Ferguson, M. B. Alice, J. Chem. Phys. 49, 2240, 1968). In the electrospray technique, analyte is dissolved in a liquid solution and sprayed from a needle. The spray is induced by the application of a potential difference between the needle and a counter electrode. The spray results in the formation of fine, charged droplets of solution containing analyte molecules. In the gas phase, the solvent evaporates leaving behind charged, gas phase, analyte ions. Very large ions can be formed in this way. Ions as large as 1 MDa have been detected by ESI in conjunction with mass spectrometry (ESMS).
For example, FIG. 1 depicts a conventional mass spectrometer using an ESI ion source. As shown, ions are introduced into ionization chamber via spray needle 10. At the end of spray needle 10, the solution is formed into a spray 12 of fine droplets. Spray 12 is formed as a result of an electrostatic field applied between spray needle 10 and sampling orifice 14. Sampling orifice 14 may be an aperture, capillary (shown), or other similar inlet leading into vacuum chamber 4 of the mass spectrometer. While in the ionization chamber 2, electrosprayed droplets evaporate thereby producing gas phase analyte ions. In addition, heated drying gas may be used to assist the evaporation of the droplets. The analyte ions are carried with the gas from ionization chamber 2 through the sampling orifice 14 and into the differential pumping system of the mass spectrometer, comprising vacuum chambers 4, 6 & 8 and pumps 20, 22, & 24. With the assistance of electrostatic lens 16 and a conventional ion guide 18, sample analyte ions pass through the vacuum system of the source (i.e., regions 4 & 6) before entering high vacuum region 8 wherein the mass analyzer (not shown) is positioned. Once in the mass analyzer, the sample ions are analyzed to produce a mass spectrum. Some of the analyzers which may be used in such a system include quadrupole, ICR, TOF, etc.
In addition to ESI, any other ion production method that can be adapted to atmospheric pressure might be used. For example, MALDI has recently been adapted by Victor Laiko and Alma Burlingame to work at atmospheric pressure (Atmospheric Pressure Matrix Assisted Laser Desorption Ionization, poster #1121, 4th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25-29, 1998) and by Standing et al. at elevated pressures (Time of Flight Mass Spectrometry of Biomolecules with Orthogonal Injection+Collisional Cooling, poster #1272, 4th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25-29, 1998; and Orthogonal Injection TOFMS Anal. Chem. 71(13), 452A (1999)). The benefit of adapting ion sources in this manner is that the ion optics and mass spectral results are largely independent of the ion production method used.
An elevated pressure ion source always has an ion production region (wherein ions are produced) and an ion transfer region (wherein ions are transferred through differential pumping stages and into the mass analyzer). The ion production region is at an elevated pressure—most often atmospheric pressure—with respect to the analyzer. The ion production region will often include an ionization “chamber” (e.g. FIG. 1, ionization chamber 4). In an ESI source, for example, liquid samples are “sprayed” into the “chamber” to form ions.
Once the ions are produced, they must be transported to the vacuum for mass analysis. Generally, mass spectrometers (MS) operate in a vacuum between 10−4 and 10−10 torr depending on the type of mass analyzer used. In order for the gas phase ions to enter the mass analyzer, they must be separated from the background gas carrying the ions and transported through the single or multiple vacuum stages.
The use of multipole ion guides has been shown to be an effective means of transporting ions through a vacuum system. Publications by Olivers et al. (Anal. Chem, Vol. 59, p. 1230-1232, 1987), Smith et al. (Anal. Chem. Vol. 60, p. 436-441, 1988) and Douglas et al. U.S. Pat. No. 4,963,736 (Douglas) have reported the use of AC-only quadrupole ion guides to transport ions from an API source to a mass analyzer. Such multipole ion guides may be configured as collision cells capable of being operated in RF only mode with a variable DC offset potential applied to all rods. Thomson et al., U.S. Pat. No. 5,847,386 (Thomson) also describes a quadrupole ion guide. The ion guide of Thomson is configured to create a DC axial field along its axis to move ions axially through a collision cell, inter alia, or to promote dissociation of ions (i.e., by Collision Induced Dissociation (CID)).
Other schemes are available utilizing both RF and DC potentials in order to facilitate the transmission of ions of a certain range of m/z values. For example, in H. R. Morris et al., High Sensitivity Collisionally Activated Decomposition Tandem Mass Spectrometry on a Novel Quadrupole/Orthogonal Acceleration Time-of-Flight Mass Spectrometer, Rapid Commun. Mass Spectrom. 10, 889 (1996) (Morris), uses a series of multipoles in their design, one of which is a quadrupole which is capable of being operated in a “wide bandpass” mode or a “narrow bandpass” mode. In the wide bandpass mode, an RF-only potential is applied to the quadrupole and ions of a relatively broad range of m/z values are transmitted. In narrow bandpass mode both RF and DC potentials are applied to the quadrupole such that ions of only a narrow range of m/z values are selected for transmission through the quadrupole. In subsequent multipoles the selected ions may be activated towards dissociation. In this way, the instrument of Morris is able to perform MS/MS experiments with the first mass analysis and subsequent fragmentation occurring in what would otherwise be simply a set of multipole ion guides.
Further, mass spectrometers similar to that of Whitehouse et al. U.S. Pat. No. 5,652,427 (Whitehouse), entitled “Multipole Ion Guide for Mass Spectrometry”, use multipole RF ion guides to transfer ions from one pressure region to another in a differentially pumped system. In the source of Whitehouse, ions are produced by ESI or APCI at substantially atmospheric pressure. These ions are transferred from atmospheric pressure to a first differential pumping region by the gas flow through a glass capillary. Ions are transferred from this first pumping region to a second pumping region through a “skimmer” by an electric field between these regions as well as gas flow. A multipole in the second differentially pumped region accepts ions and guides them through a restriction and into a third differentially pumped region. This is accomplished by applying AC and DC voltages to the individual poles.
A four vacuum stage ES/MS quadrupole mass spectrometer according to Whitehouse, incorporating a multipole ion guide beginning in one vacuum pumping stage and extending contiguously into an adjacent pumping stage, is depicted in FIG. 2. As discussed above, ions are formed from sample solution by an electrospray process when a potential is applied between spray needle 27 of sprayer 26 and sampling orifice 38. According to the prior art system shown in FIG. 2, capillary 60 is used to transport ions from atmospheric pressure region 28, where the ions are formed, to first pumping region 30. Lenses 62 and 56 are used to guide the ions from exit end 40 of capillary 60 to a fourth pumping region 36 containing a mass analyzer. In this case, a reflectron TOF mass analyzer is shown. Between lenses 62 and 48, RF only hexapole ion guide 42 is used to guide ions through differential pumping stages 32 and 34 to exit end 46 of ion guide 42 and into mass analysis region 36 through orifice 50. The hexapole ion guide 42 according to this prior art design is intended to provide for the efficient transport of ions from one location (i.e., the entrance 58 of skimmer 56) to a second location (i.e., orifice 50). FIG. 2 is a diagram of a four vacuum pumping stage orthogonal pulsing API/MS system with a reflectron Time-Of-Flight mass analyzer. For the purpose of illustration, an electrospray ion source is shown as the API source. This could alternatively be an APCI or an ICP source. Sample bearing liquid is introduced through the electrospray needle 26 and is electrosprayed (with or without pneumatic assistance) into chamber 28 as it exits the needle at 27. The charged droplets produced evaporate and desorb gas phase ions both in chamber 28 and as they are swept into the vacuum of a mass spectrometer through the annulus in capillary 60. A portion of the ions that enter the first vacuum stage 30 through the capillary exit 40 are focused through the orifice 58 in skimmer 56 with the help of lens 62 and the potential set on the capillary exit 40. Ions passing through skimmer orifice 58 enter the multipole ion guide 42 which begins in vacuum pumping stage 32 and extends unbroken into vacuum stage 34. If the multipole ion guide AC and DC voltages are set to pass ions falling within a range of m/z then ions within that range which enter the multipole ion guide will exit at 46 and are focused with exit lens 48 through the TOF analyzer entrance orifice 50. This primary ion beam 82 passes between electrostatic lenses 64 and 68 located in the fourth pumping stage 36. The relative voltages on lenses 64, 68 and 70 are pulsed so that a portion of the ion beam 82 falling in between lenses 64 and 68 is ejected as a packet through grid lens 70 and accelerated down flight tube 80. The ions are steered by x and y lens sets diagrammatically illustrated by 72 as they continue moving down flight tube 80. As shown in this illustrative configuration, the ion packet is reflected through a reflectron or ion mirror 78 and detected at detector 74. As a pulsed ion packet proceeds down flight tube 80, ions with different m/z separate in space due to their velocity differences and arrive at the detector at different times. The use of orthogonal pulsing in an API/TOF system helps to reduce the ion energy spread of the initial ion packet allowing for the achievement of higher resolution and sensitivity. Also disclosed by Whitehouse is the use of collisional gas within hexapole ion guide 42 to cool the ions to thermal velocities through collisional cooling.
In the scheme of Whitehouse, an RF only potential is applied to multipole ion guide 42. As a result, ion guide 42 is not “selective” but rather transmits ions over a broad range of mass-to-charge (m/z) ratios. Such a range as provided by prior art multipoles is inadequate for certain applications, such as for Matrix Assisted Laser Desorption/Ionization (MALDI), because the ions produced may be well out of this m/z range. In other words, high m/z ions such as are often produced by the MALDI ionization method are often out of the range of transmission of conventional multipole ion guides.
Thus, electric voltages usually applied to the conventional ion guide are used to transmit ions from an entrance end to an exit end. Analyte ions produced in the ion production region pass through a capillary or other ion transfer device to move the ions to a differentially pumped region and enter the ion guide at the entrance end. Through collisions with gas in the ion guide, the kinetic energy of the ions is reduced to thermal energies. Simultaneously, the RF potential on the poles of the ion guide forces ions to the axis of the ion guide. Then, ions migrate through the ion guide toward its exit end, where the ions typically either enter a second ion guide or enter the mass analysis region.
Whitehouse also discloses use of two or more ion guides in consecutive vacuum pumping stages to allow different DC and RF values. However, losses in ion transmission efficiency may occur in the region of static voltage lenses between ion guides. For example, a commercially available API/MS instrument manufactured by Hewlett Packard incorporates two skimmers and an ion guide. The drag stage of a conventional turbo pump is used to pump the region between the skimmers. That is, an additional pumping stage/region is added without the addition of an extra turbo pump, and therefore, improved pumping efficiency may be achieved. In this dual skimmer design, there is no ion focusing device between skimmers, therefore ion losses may occur as the gases are pumped away. A second example is demonstrated by a commercially available API/MS instrument manufactured by Finnigan which applies an electrical static lens between capillary and skimmer to focus the ion beam. Due to narrow mass range of the static lens, the instrument may need to scan the voltage to optimize the ion transmission.
In addition, the electrode rods of the prior art multipole ion guides described above are positioned in parallel and are equally spaced at a common radius from the centerline of the ion guide. Thus, ions with a m/z ratio falling within the ion guide stability window established by the applied voltages have stable trajectories within the ion guide's internal volume bounded by the parallel, evenly spaced rods. This is true for quadrupoles, hexapoles, etc. For example, FIGS. 3A & 3B depict a prior art hexapole ion guide 88. Ion guide 88 shown comprises six conducting rods 86 aligned in parallel and adjacent to one another to form a cylinder-like structure. That is, six parallel conducting rods 86 are evenly spaced from centerline 90 (or axis) of ion guide 88. At either end of ion guide 88 are positioned DC electrodes—skimmer 84 at entrance end 92 and gate electrode 89 at exit end 94.
During operation, DC potentials are applied to each of skimmer 84 and gate electrode 89 of multipole ion guide 88 (shown is a hexapole). At skimmer 84 (i.e., entrance end 92 of ion guide 88), ions pass from an ion source region (not shown) through electrically conducting skimmer 84 into the region between the parallel conducting rods 86. In other words, the DC potential applied to skimmer 84 is set such that the ions are focused into ion guide 88. Next, a high voltage RF potential is applied to conducting rods 86 of ion guide 88 to “force” the ions (or focus the ions) to centerline 90 (or axis) of the ion guide. In addition, a collisional gas has been used within such ion guides to collisionally cool the ions therein. Next, the ions will migrate toward exit end 94 of ion guide 88, and at exit end 94 gate electrode 89 is positioned such that a repulsive DC potential may be applied to trap the ions within ion guide 88 until it is time to analyze the ions. On the other hand, when a non-repulsive DC potential is applied to gate electrode 89, the ions may pass freely out of ion guide 88 and into a mass analyzer.
In sum, previous ion guides (e.g., quadrupoles, hexapoles, etc.) have comprised parallel conducting rods evenly spaced from a centerline, having DC electrodes positioned at their entrance and exit ends, and high voltage RF and DC potentials are applied thereto to focus, transmit, and/or trap ions. It has been observed that such ion guides are limited in their applications. Specifically, such conventional ion guides may only accept ions from a single ion production means and changing from one ion production means to another is cumbersome and time consuming. In addition, prior art ion guides are often inadequate for transmission of ions produced by the MALDI method, as these ions are often of a m/z range out of the range for which the ion guides are capable. Yet another disadvantage of prior art ion guides is their limited use for mass selection and performing chemical reactions. As discussed below, the ion guide of the present invention overcomes these limitations and/or deficiencies in conventional ion guides.